When primers and paints are used over untreated wood, a problem is often encountered in which tannin from the surface of the wood is seen to bleed through the paint and discolor the finish. Tannin is not a single compound but a complex group of compounds that occur naturally in wood and other plant materials. Since the chemical structures of the tannins vary from one species of wood to another, the intensity of the color of the tannin and the severity of the problem of tannin-stain bleed will likewise vary with the species of wood used. Cedar and redwood are two common types of wood subject to severe tannin-stain bleed.
As used in the art, the term "tannin-stain bleed" refers to the complexation of a transition metal ion by a tannate ion derived from tannin. In particular, largely composed of acidic compounds, tannins will dissolve in alkaline liquids (such as water-based paints) to form water-soluble tannate salts, which are known to form highly-colored water-soluble or water-dispersible complexes with certain transition-metal ions, such as ferric ions. These tannate salts and complexes can diffuse through the paint film before it has initially dried and also can be carried through the water-permeable dried paint film by rainwater.
There are a number of paint additives that are commonly used in an effort to control the problem of tannin-stain bleed. All of these additives contain cations that will form lightly-colored water-insoluble compounds with the tannate ions to prevent their migration through the paint film. For many years, pigments containing lead compounds were used for this purpose, as described in U.S. Pat. No. 3,214,398 (the disclosure of which is incorporated herein by reference); however, the toxicity of lead compounds has virtually eliminated their use for this purpose. U.S. Pat. No. 4,218,516 describes the use of magnesium hydroxide to form a leach-resistant magnesium hydroxide-tannin complex. U.S. Pat. Nos. 2,818,344; 3,085,893; and 3,033,700 (the disclosures of which are incorporated herein by reference) describe the use of barium metaborate to prevent tannin-stain bleed as well as other types of discoloration. With this compound the barium ions would form an analagous barium-tannin complex that would be insoluble in water. Other pigments containing alkaline earth metals (such as calcium barium phosphosilicate as well as other calcium compounds or strontium compounds) would be expected to react with and precipitate tannins in a similar manner.
Water-soluble basic aluminum salts (such as halides, nitrates, formates, sulfates, and phenolsulfonates as well as double salts containing these anions) are used in another type of paint additive for tannin-stain blocking. The use of these basic aluminum salts for this purpose is disclosed in U.S. Pat. Nos. 3,900,620 and 4,021,398.
Aluminum hydroxide is known to be amphoteric and is cationic in the basic aluminum salts described in these patents. In this cationic form, basic aluminum hydroxide would precipitate the tannate anions on the surface of the wood and thus prevent tannin-stain bleed.
S-aryl cycloaliphatic sulfonium compounds are described in U.S. Pat. No. 3,900,619 as additives for the prevention of tannin-stain bleed. Applied in this manner, the sulfonium cations would form insoluble ion pairs with the tannate ions on the surface of the wood, thus immobilizing the tannin.
All of these additives use organic or inorganic cations to form water-insoluble precipitates with the tannate ions on the surface of the wood to prevent tannin-stain bleed. It is not believed that the art has recognized that much of the tannin-stain color could arise from the formation of water-soluble or water-dispersible complexes of transition-metal ions (such as ferric ions) with the tannate ions.
The color of the ferric-tannin complex is so intense that only a trace amount of the metal ion would be necessary to produce a visible stain on a white paint film. Trace (ppm) levels of ferric ions can be introduced from a number of sources, including the wood itself as well as the paint. Corrosion of metal fasteners such as nails or screws can also be sources of trace amounts of ferric ion. All of the conventional tannin-stain blocking agents discussed above are used to control the solubility of the tannin but do not control the concentration of trace amounts of ferric ion or other transition-metal ions in the paint film or on its surface.